The present invention relates to a method of applying, by a crosshead extruder, an outer layer of an unvulcanized rubber mass or a similar coating material onto a cylindrical workpiece, e.g. for the production of stabilized rubber cylinders for use in the printing industry. The term "crosshead" refers to an arrangement whereby an elongate carrier tube is conveyed through a "crosshead" extruder device which receives the coating material under a high pressure through a side inlet thereof and compacts this material on and around the carrier tube workpiece as the latter leaves the crosshead through an extruder nozzle adjacent the front end of the crosshead. Thus, inside the crosshead the crosswise introduced coating material is distributed around the cylindrical workpiece and is forced into contact therewith adjacent the outlet nozzle of the device, whereby the workpiece may leave the nozzle coated with a surface layer of any desirable thickness as determined by the diameter of the extruder nozzle opening.
However, a highly viscid rubber mass is difficult to handle, and it has been found that these simple crossheads are not usable for producing coated products when the diameter of the articles to be coated exceeds some 50 mm. One problem is that the rubber is not well distributed around the workpiece tube, and it has been proposed in, for example, GB-A-1,082,538, to effect a better distribution by incorporating in the crosshead a stationary worm structure which will guide the rubber around the workpiece, but still such a system will be usable for small diameters only, as it will be practically impossible to force the rubber through a worm groove of any considerable length.
In connection with less viscid materials it is known to effect an extrusion by a crosshead having a rotating worm with a central passage for the throughlet of the articles to be coated, whereby the worm operates to draw the material from a supply hopper and deliver it under pressure adjacent the nozzle end of the crosshead. Such an operation will not be applicable for highly viscid materials, not only because they cannot just be drawn from a hopper, but also because the rotating discharge end of the work will cause the material to be almost threaded onto the article to be coated, and normally the quality of such an extrusion will be unacceptable.
Generally, in order to obtain a good result the space just inside the extruder nozzle and around the article to be coated should be filled by the viscid material under a high pressure and with an entirely even distribution of the material. In a crosshead structure the correct filling of the space can be promoted by an increased inlet pressure of the material or an increased axial distance between the material inlet port and the extruder nozzle, but such measures will be usable to a limited extent only, because for larger dimensions the required movements of the material will be connected not only with extremely high pressure requirements, but also with the development of a considerable compression heat and frictional heat in the material. This, of course, is acceptable only to a certain extent, due to the risk of the rubber starting to be vulcanized in an uncontrolled manner.
In practice, therefore, despite many prior proposals for crosshead extruders of different kinds, it is customary to produce rubber coated cylinders of diameters larger than some 5-10 cm by entirely different and rather difficult methods, primarily by a rolling on of thin rolled rubber sheets, this with the well known risk of air blisters being trapped between the sheets in an unacceptable manner.
It is the purpose of this invention to provide a method whereby even large diameter workpieces may be coated by a highly viscid material such as rubber, based on the use of a crosshead extruder.
According to the invention the material is supplied under a high pressure to a crosshead extruder having a centrally apertured rotatable worm, which is operable to convey the material towards the extruder nozzle past a cylindrical worm core head, which projects towards the extruder nozzle from the front end of the core of the worm and is rotatably connected with the worm, the material being fed to the extruder nozzle by powered rotation of the worm while the worm core head is held substantially non-rotating relative to the extruder nozzle. The invention is based on the recognition that while a power driven worm is not by itself, in the present context, a usable means for picking up the material and building up a high material pressure adjacent the extruder nozzle, it will nevertheless be a perfect means for transporting and distributing an already pressurized material, which is forced into the crosshead. The material will be moved by the worm due to the powered rotation thereof, this requiring a much smaller inlet pressure on the material and giving rise to a much smaller friction heat generation than if the transfer of the material should take place through fixed channel means. Of course, the worm may well contribute to a high material pressure adjacent the extruder nozzle, but the more important aspect of its use is that it does away with the discussed limitation as to the length of the transfer path of the highly viscid material, such that it will allow for the production of coated articles practically regardless of their size.
As already mentioned, the use of a rotating worm, regardless of the more detailed purpose thereof, is liable to destroy the desired uniformity of the coating, because it will tend to "thread38 the highly viscid material out through the extruder nozzle. For this reason it is very important that the worm core head is arranged in a generally non-rotating manner, as this will brake the threading of the material and thus eliminate the associated drawback of the use of a worm.
The invention also comprises a novel crosshead for effecting the disclosed method. The required supply of the material to the crosshead at high pressure can be effected by known means, just as for the supply to ordinary crossheads for highly viscid materials, but the requirement as to the non-rotating worm core head will involve a novel construction of the crosshead, even compared to crossheads for less viscid materials, in which the use of a rotating worm is known for the picking up of the material from a hopper and for the building up of the required pressure at the extruder nozzle. In crossheads for such materials there is no specific problem with respect to the uniformity of the material as applied as a coating onto the articles to be coated, because the discussed threading-effect is very little pronounced, if existing at all, so for this reason there is no need to arrange for any non-rotating worm core head.
It should be mentioned, however, that it has nevertheless been proposed to provide a crosshead of the latter type with a non-rotating worm core head, though for an entirely different purpose. In this connection, German Patent Specification No. 545.937, proposes a presumably non-rotating worm core head which is held in a centered position by radial adjustment screws. The purpose of this arrangement is to make possible a fine centering of the workpiece inside the extruder nozzle as achievable by an adjustment of the radial screws. However, if used in the present connection, with rubber or a similar highly viscid material being moved from the end of the worm towards the extrusion nozzle the radial screws might give rise to a shadow effect in the material, which could not thereafter flow together into a fully homogenous mass adjacent the extruder nozzle. This is a theoretical consideration, because the first result would be that the radial adjustment screws would break as matches when subjected to the pressure of the highly viscid material. If they were made strong enough to resist this pressure they would be much thicker, and their shadowing effect would be still more pronounced, i.e. such an arrangement is simply not usable in connection with the present invention.
In the crosshead according to the invention the worm core head as protruding from the delivery end of the rotating worm is anchored against rotation in being connected with a holding pipe, which extends rearwardly through the central passage of the worm, closely inside the inner wall thereof and rearwardly beyond the rear end of the worm, the holding pipe at its rear end, outside the worm, being non-rotatably anchored to the housing of the crosshead. Hereby the worm core head is stabilized against rotation without having any portions protruding into the moving path of the viscid material, while still allowing cylindrical workpieces to be moved through the crosshead in a centered position therein as determined by the inner passage of the holding pipe.
Of course, the interior diameter of the holding pipe should be adpated to the size of the workpieces to be handled, or be provided with a lining tube so adapted, such that an exact centering of the workpieces will be possible.
It is a very important possibility that, in the crosshead according to the invention, the worm core head or nozzle core may be axially adjustable from the rear end of the holding tube. The axial distance between the nozzle core and the extruder nozzle may be rather critical for a good result, and both for different diameters and different coating materials such an axial adjustability will be highly advantageous.
In practice it is desirable that the nozzle core has a flange portion sealingly engaging the front end of the worm core portion, and, for this reason, it is not practical to use an axial adjustable nozzle core if an associated result is that this axial engagement is relieved. However, the engagement need not be relieved when the holding tube is combined with an inner lining tube, which projects in front of the nozzle core and is axially adjustable from a free rear end portion adjacent or outside the rear end of the holding tube. The nozzle core may then be suitable shaped as a rigid cone sealing rearwardly against the front end of the worm core portion and narrowing towards the extruder nozzle, while the distance between the extruder nozzle and the effective front end of the nozzle core will be variable with the position of the front end of the axially adjustable lining tube.